Is there a way to programmatically set an interface MTU using C in Linux?

Question

At the moment my program is making a system() call to ifconfig to do this.

It seems a bit messy - maybe ifconfig is not on the path, or in some non-standard location. And then I'd need to check for the iproute2 equivalent in case of failure.

Is there a way to set this programmatically using C?

askmish · Accepted Answer · 2014-07-31T06:15:28.453

5

You can set the SIOCSIFMTU field in an ioctl call, something like this:

struct ifreq ifr; 
ifr.ifr_addr.sa_family = AF_INET;//address family
strncpy(ifr.ifr_name, "eth0", sizeof(ifr.ifr_name));//interface name where you want to set the MTU
ifr.ifr_mtu = 9100; //your MTU size here
if (ioctl(sockfd, SIOCSIFMTU, (caddr_t)&ifr) < 0)
  //failed to set MTU. handle error.

The above code will set the MTU of a device(as in ifr.name) using ifr_mtu field in the ifreq structure.

Refer: http://linux.die.net/man/7/netdevice

edited Jul 31 '14 at 06:15

answered Jul 30 '14 at 04:39

askmish

6,464
23
42

1

Thanks, that works! Any idea why this needs a socket fd? I always thought the MTU is a property of a link, rather than a socket. – James Jul 30 '14 at 12:18
1

Quote from the man page: "Linux supports standard ioctls to configure network devices. They can be used on any socket's file descriptor regardless of the family or type. They pass an ifreq structure." Refer: http://linux.die.net/man/7/netdevice – askmish Jul 31 '14 at 06:09

score 4 · Answer 2 · answered Dec 04 '19 at 10:28

You can also use netlink. The full example:

#include <stdio.h>
#include <string.h>
#include <net/if.h>
#include <sys/socket.h>
#include <linux/rtnetlink.h>

#define IFACE_NAME "enp5s0"

int main(void) {
     int ret, nl_sock;
     unsigned int mtu = 8000;
     struct rtattr  *rta;
     struct {
          struct nlmsghdr nh;
          struct ifinfomsg  ifinfo;
          char   attrbuf[512];
     } req;

     nl_sock = socket(AF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);
     if(nl_sock < 0) {
          perror("socket():");
          return -1;
     }

     memset(&req, 0, sizeof req);
     req.nh.nlmsg_len   = NLMSG_LENGTH(sizeof(struct ifinfomsg));
     req.nh.nlmsg_flags = NLM_F_REQUEST;
     req.nh.nlmsg_type  = RTM_NEWLINK; // RTM_SETLINK
     req.ifinfo.ifi_family = AF_UNSPEC;
     req.ifinfo.ifi_index  = if_nametoindex(IFACE_NAME);
     if(!req.ifinfo.ifi_index) {
          perror("if_nametoindex():");
          return -1;
     }
     printf("%s index: %d\n", IFACE_NAME, req.ifinfo.ifi_index);
     req.ifinfo.ifi_change = 0xffffffff;
     rta = (struct rtattr *)(((char *) &req) + NLMSG_ALIGN(req.nh.nlmsg_len));
     rta->rta_type = IFLA_MTU;
     rta->rta_len = RTA_LENGTH(sizeof(unsigned int));
     req.nh.nlmsg_len = NLMSG_ALIGN(req.nh.nlmsg_len) + RTA_LENGTH(sizeof mtu);
     memcpy(RTA_DATA(rta), &mtu, sizeof mtu);

     ret = send(nl_sock, &req, req.nh.nlmsg_len, 0);
     if(ret < 0) {
          perror("send():");
          return -1;
     }

     return 0;
}

Is there a way to programmatically set an interface MTU using C in Linux?

2 Answers2